Cholesterol and calcium, the latter in the form of hydroxyapatite, accumulate in atherosclerotic lesions. We have demonstrated that these organic and inorganic constituents of lesions can accumulate together, closely associated in composite crystals. Using the fluorescent cholesterol probe, filipin, we identified unesterified cholesterol that was associated with calcium (i.e., hydroxyapatite) granules in tissue sections of lesions and with calcium granules isolated from lesions. We also have shown with filipin staining that cholesterol can associate with pre-formed hydroxyapatite crystals, in vitro. Scanning electron microscopy coupled with energy dispersive X-ray analysis confirmed the physical association of many small crystallites of cholesterol with larger crystals of hydroxyapatite. Our results support the hypothesis of Craven that because of the similarities in crystal structure of hydroxyapatite and cholesterol, a microcrystal of either may serve as a nidus for the growth of the other. Our studies suggest that this or a related process occurs within atherosclerotic lesions. The extent to which this process contributes to cholesterol and calcium deposition within atherosclerotic lesions remains to be determined.